A hydraulic drive system for use in construction machines such as hydraulic excavators comprises a hydraulic pump, a plurality of hydraulic actuators driven by a hydraulic fluid supplied from the hydraulic pump, and a plurality of directional control valves for controlling respective flow rates of the hydraulic fluid supplied from the hydraulic source to a plurality of actuators.
From the standpoint of reducing energy consumption primarily, it is proposed in a hydraulic drive system of that type to employ a load sensing control technique for controlling a delivery pressure of the hydraulic pump dependent on the load pressure. As examples of such a hydraulic drive system, there are known GB 2,195,745A, DE 2,906,670A1, U.S. Pat. No. 4,939,023, etc. To carry out the load sensing control, those examples of the prior art employ a pump flow controller for controlling a delivery rate of the hydraulic pump so that the delivery pressure of the hydraulic pump is held higher by a fixed value than a maximum load pressure among the plurality of actuators. The plurality of directional control valves each comprises a pump port, a pressure chamber capable of communicating with the pump port, a feeder passage capable of communicating with the pressure chamber, an actuator port capable of communicating with the feeder passage, a reservoir port capable of communicating with the actuator port, a first meter-in variable restrictor disposed between the pump port and the pressure chamber, and a pressure compensating valve having a pair of opposite ends, one of which is subjected to a pressure in the pressure chamber and the other of which is subjected to the maximum load pressure among the plurality of actuators. With the pair of opposite ends respectively subjected to the pressure in the pressure chamber and the maximum load pressure, as mentioned above, the pressure compensating valve serves to control the pressure in the pressure chamber dependent on the maximum load pressure for holding the differential pressure across the meter-in variable restrictor at a fixed value, during the combined operation in which plural actuators are driven simultaneously. The differential pressures across the meter-in variable restrictors of all the directional control valves are thereby made equal to one another so that the flow rate of the hydraulic fluid from the hydraulic pump is distributed in accordance with the ratio of opening area between the variable restrictors to perform the desired combined operation.
Of the prior art, the apparatus disclosed in U.S. Pat. No. 4,939,023 is arranged such that one of the directional control valves comprises a pressure reducing valve disposed between the pressure compensating valve and the actuator port for reducing the pressure of the hydraulic fluid supplied to the associated actuator, a load line for leading out the load pressure via a fixed restrictor, and a proportional pressure relief valve of which relief setting pressure is regulated by a pilot pressure from a control lever unit to limit the pressure in the load line, the pressure in the load line being led to act on a setting sector of the pressure reducing valve to thereby control an outlet pressure of the pressure reducing valve dependent on the setting pressure of the proportional pressure relief valve.
The above examples of the prior art have, however, the following problems.
In the hydraulic drive systems disclosed in the above-cited GB 2,195,745 and DE 2,906,670A1, when a control lever for the directional control valve is manipulated to operate the associated actuator, the hydraulic fluid is momentarily forced to flow at a flow rate corresponding to the resultant opening of the meter-in variable restrictor of the directional control valve. Accordingly, upon the control lever being quickly manipulated, the actuator is abruptly operated. This raises a problem in the case of driving a member of large inertia such as a swing of a hydraulic excavator, for example. More specifically, while the flow rate is abruptly increased upon the control lever of the directional control valve being quickly manipulated, the swing to be driven by a swing motor has large inertia and, therefore, the pressure in the system reaches the relief pressure set for limiting a maximum value of the circuit pressure. In this event, the prior art can no longer effect the pressure control and an acceleration of the swing as an inertial body is maximized, causing an operator to feel a shock. This also practically holds true in the case of traveling, boom-up and so forth other than the swing.
Further, in the aforementioned hydraulic drive system, when a tilting angle of the hydraulic pump is changed to a small extent, the flow rate of the hydraulic fluid delivered from the hydraulic pump is also changed and so is the sensing pressure, i.e., the maximum load pressure. If the amount of such a change is large, the delivery rate of the hydraulic pump is changed again to a large extent, which may cause oscillation in the circuit as a result of repetitions of the above process.
On the other hand, with the prior art disclosed in U.S. Pat. No. 4,939,023, the pressure of the hydraulic fluid supplied to the actuator is reduced in response to the pilot pressure at start-up of the swing, thereby preventing the swing motor from being abruptly operated. Also, even when the delivery rate of the hydraulic pump is slightly fluctuated, the load pressure of the swing motor will not fluctuate, because the setting of the proportional pressure relief valve is fixed and so is the setting of the pressure reducing valve as long as the operation amount of the control lever is kept fixed. It is thus possible to suppress change in the load sensing pressure caused by slight fluctuations in the pump delivery rate. However, this prior art has the following problem.
When the swing starts its inertial rotation after start-up thereof, the load pressure of the swing motor is reduced. If the load pressure lowers below the setting pressure of the pressure reducing valve, the latter valve can no longer effect its function. Under that condition, when the delivery rate of the hydraulic pump is slightly fluctuated as mentioned before, the load pressure of the swing motor is changed and so is the load sensing pressure, which may cause oscillation in the circuit, as with the foregoing prior art.
There is generally such a tendency that when the load pressure is changed so as to increase during the operation of an actuator, vibration of the actuator is damped if the flow rate of the hydraulic fluid supplied to the actuator is reduced, continues if it remains the same, and is brought into oscillation if it is increased. With the prior art disclosed in U.S. Pat. No. 4,939,023, since the proportional relief valve is closed under a condition that the load pressure of the swing motor is reduced below the setting pressure of the pressure reducing valve, no part of the hydraulic fluid passing through the directional control valve is now discharged into a reservoir (tank) via the proportional relief valve. In other words, all of the hydraulic fluid passing through the directional control valve is supplied to the actuator. Further, there is no flow of the hydraulic fluid reaching the load line through the fixed restrictor, the pressure in the load line becomes equal to the load pressure so that the differential pressure across the directional control valve is controlled to be constant as usual through the load sensing control of the hydraulic pump, thus rendering constant the flow rate of the hydraulic fluid passing through the directional control valve. Accordingly, when the load pressure is changed so as to increase during the operation of an actuator as mentioned above, the flow rate of the hydraulic fluid supplied to the actuator remains the same. As a result, load fluctuations will not be damped once occurred, which may impair the working efficiency.
It is an object of the present invention to provide a hydraulic drive system and a directional control valve for use in construction machines, which can realize pressure control while maintaining adequate distribution of flow rates, prevent abrupt operation of an actuator adapted for driving an inertial body, and further suppress vibration produced in a circuit even when any of the pump delivery rate and the load pressure is fluctuated.